Portable telephones, as with other portable electrical apparatus operate from an internal, removable, power supply generally in the form of a rechargeable battery pack. Typically, the battery pack may comprise six nickel cadmium (NiCd) cells.
Work is continuously being carried out in developing new cells for battery packs, which provide smaller, more lightweight, more efficient and generally more versatile power supplies. Recently developed cells use a polymer electrolyte. One such cell is called a Lithium Solid State (LSS) cell.
LSS cells typically are of a layered structure comprising a lithium metal electrode, a polymer electrolyte, a carbon collector, and, a metallic electrode, for example, of aluminium or nickel foil. This layered structure is, typically, of the order of less than 1 mm thickness. To provide the required voltage output, a number of the layered structures--in the form of rectangular sheets--may be stacked and suitably interconnected. LSS cells are described in Electronics Times, Power Special Report, 16 Jul. 1992, pages 21-22. LSS cells have the advantage of being lightweight, small and rechargeable time after time, and, because they are in the form of very thin laminated structures can be designed into virtually any shape.
Despite these advantages, one drawback is that their voltage output is proportional to the charge remaining i.e. they exhibit a noticeable voltage drop during use, which makes them not entirely suitable for use in portable electrical apparatus. For example, a two cell battery will have an output of around 6.3 V at full charge and a 3.8 V output when discharged, but between these points, the output drops noticeably over time as shown in FIG. 1A. The discharge rate is generally linear, the exact rate being dependant upon the load coupled to the battery. This is unlike a conventional NiCd battery which exhibits an initial sharp voltage drop, with a substantially more constant voltage output thereafter until, at discharge, there is another sharp drop in output voltage.
To overcome the problem exhibited by LSS batteries, a Direct Current-to-Direct Current (DC/DC) conversion system could be provided in the electrical apparatus to provide a stable output voltage before it is supplied to the electronic circuitry of the apparatus. FIG. 1B shows how the voltage output from an LSS battery can be stabilized as a function of time by providing such a DC/DC conversion system. However, this has the problem of electromagnetic interference (EMI), i.e. the electromagnetic radiation emitted by the DC/DC converter that "disturbs" other circuitry in the apparatus e.g. the radio frequency (RF) section of a telephone's electronic circuitry.
In addition, providing a DC/DC converter in the apparatus is an additional cost when it may not be needed, for example, if the apparatus is also capable of using a conventional NiCd battery as its power source. It also means that LSS battery cannot be used in "older" apparatus which do not have a DC/DC converter.